1. Field of the Invention
The present invention relates to a driving device using an electromechanical transducing element, a driving circuit for the device, and an apparatus having the device and the circuit. Specifically, the invention relates to an apparatus such as an X-Y table, a camera, an overhead projector, or a binocular, a driving device for such an apparatus, and a driving circuit for the device.
2. Description of the Prior Art
Conventionally, an X-Y table, a taking lens of a camera, or the like is driven by a driving device using an electric motor. It has been pointed out that such a device is large in size and has disadvantages such as generation of a magnetic field and noises. Therefore, a driving device using an electromechanical transducing element has been proposed as a means which can solve such various problems.
As shown in a driving principle view of FIG. 1(A), for example, the device comprises: a fixed member B which is fixed to a base (not shown); a piezoelectric element A in which one end face in the expansion and contraction directions is secured to the fixed member B; a driving shaft S which is supported onto the base or the fixed member B so as to be movable in the expansion and contraction directions of the piezoelectric element A, and in which an axial end face is fixedly coupled to the other end face in the expansion and contraction directions of the piezoelectric element A; and a moving member M which is frictionally coupled to the driving shaft S. A pulse voltage of an approximately saw-tooth waveform such as that shown in FIG. 1(B) is applied to the piezoelectric element A, so that the piezoelectric element A expands or contracts fast in one direction and slow in the opposite direction, thereby causing the driving shaft S to be moved fast in the one direction and slow in the opposite direction. As a result, the moving member M is driven along the driving shaft S.
Specifically, when the voltage applied to the piezoelectric element A is suddenly raised from a reference voltage indicated by (1) in FIG. 1(B) to a maximum voltage as indicated by (2), the piezoelectric element A expands from the reference state shown in (1) of FIG. 1(A) abruptly or fast as shown in (2), and the driving shaft S is moved fast in the direction along which the shaft is separated from the fixed member B. At this time, the acceleration of the driving shaft S is so high that the inertia force of the moving member M is larger than the friction force exerted between the moving member M and the driving shaft S, and hence the driving shaft S is moved while the moving member M is not substantially moved.
When the voltage applied to the piezoelectric element A is thereafter lowered slow from the maximum voltage to the reference voltage as indicated by (3) in FIG. 1(B), also the driving shaft S is moved slow in accordance with the lowering as shown in (3) of FIG. 1(A). At this time, the acceleration of the driving shaft S is so low that the inertia force acting on the moving member M is small. Therefore, the friction force exerted between the moving member M and the driving shaft S causes the moving member M to be moved slow integrally with the driving shaft S.
When such a pulse voltage of an approximately saw-tooth waveform is continuously applied to the piezoelectric element A, the moving member M is moved in the forward direction. In the following, description will be made assuming that the left portion of each figure is in the front side and the right portion is in the rear side.
When the moving member is to be moved in the backward direction, a pulse voltage which is opposite in polarity to that in the case of the forward direction is applied to the piezoelectric element A so as to invert the relationships between the expansion and contraction directions of the piezoelectric element A and the fast and slow movements. As shown in (4) to (6) of FIGS. 1(A) and 1(B), for example, the pulse voltage has an approximately saw-tooth waveform which abruptly falls from the reference voltage to the minimum voltage and then rises slow from the minimum voltage to the reference voltage.
In order to apply a pulse voltage of such a waveform to the piezoelectric element A, a driving circuit shown in FIG. 2 is used. The driving circuit generally comprises: four switches respectively configured by transistors FET1 to FET4; two constant-current circuits I1 and I2; and six terminals to which a high-level signal (hereinafter, referred to as "H signal") or a low-level signal (hereinafter, referred to as "L signal") is adequately supplied from a controller (not shown). A pulse voltage of a saw-tooth waveform is applied across terminals P1 and P2 of the piezoelectric element A.
As shown in a timing chart of FIG. 3, when the driving operation is to be conducted in the forward direction, for example, the H signal is supplied to FORWARD terminal to turn on the transistor FET2, thereby setting one terminal P1 of the piezoelectric element A to the ground level. Next, the H signal is supplied to FAST_F terminal to turn on the transistor FET3, so that a voltage V.sub.HV of HV terminal is applied to the other terminal P2 of the piezoelectric element A. At this time, the driving voltage Vact which is seen from the one terminal P1 of the piezoelectric element A toward the other terminal P2 rises suddenly from the ground level to the voltage V.sub.HV, and the piezoelectric element A is rapidly charged so as to fast extend. The L signal is then supplied to FAST_F terminal to turn off the transistor FET3, and the H signal is thereafter supplied to SLOW_F terminal to turn on the constant-current circuit I2, so that charges are removed slow through the other terminal P2 of the piezoelectric element A. As a result, the driving voltage Vact of the piezoelectric element A falls slow and the piezoelectric element contracts slow. Thereafter, the H signal is alternatingly supplied to FAST_F and SLOW_F terminals, with the result that the driving voltage Vact of the piezoelectric element A has a waveform which reciprocates in an approximately saw-tooth like manner between V.sub.HV and an intermediate level V.sub.T which lies between V.sub.HV and the ground level. Namely, the waveform of the driving voltage Vact rises abruptly when the H signal is supplied to FAST_F terminal, and falls slow when the H signal is supplied to SLOW_F terminal. When the driving operation is to be stopped, the L signal is supplied to FORWARD terminal.
When the driving operation is to be conducted in the backward direction, similarly, the H signal is supplied to BACKWARD terminal to turn on the transistor FET4, thereby setting the other terminal P2 of the piezoelectric element to the ground level. Next, the H signal is supplied to FAST_B terminal to turn on the transistor FET1, so that the voltage V.sub.HV of HV terminal is applied to the one terminal P1 of the piezoelectric element A. At this time, the driving voltage Vact of the piezoelectric element A falls suddenly from the ground level to -V.sub.HV, and the piezoelectric element A is rapidly charged so as to contract fast. The L signal is then supplied to FAST_B terminal, and the H signal is thereafter supplied to SLOW_B terminal to turn on the constant-current circuit I1, so that charges are removed slow through the one terminal P1 of the piezoelectric element A. As a result, the driving voltage Vact rises gently and the piezoelectric element extends slow. Thereafter, the H signal is alternatingly supplied to FAST_B and SLOW_B terminals, with the result that the driving voltage Vact of the piezoelectric element A has a waveform which reciprocates in an approximately saw-tooth like manner between -V.sub.HV and an intermediate level -V.sub.T which lies between -V.sub.HV and the ground level.
In the driving circuit, when the H signal is supplied at the first time to FAST_F terminal after the H signal is supplied to FORWARD terminal, i.e., immediately after the start of the driving operation, the piezoelectric element A is rapidly charged so as to suddenly start to deform from the still state. At this time, abrupt vibration occurs in the piezoelectric element A to generate a sound or noise. Even after the driving operation is ended, charges remain to exist in the piezoelectric element A. In the case where the driving operation is thereafter conducted in the opposite direction, therefore, the polarity of the piezoelectric element A is suddenly inverted. Consequently, a large potential difference is abruptly produced in the piezoelectric element A, so that a sound is generated also in this case. The piezoelectric element A may be sometimes destroyed depending on the potential difference.
By contrast, even in the case where the piezoelectric element A is charged slow and then discharged fast, when the driving operation is conducted with the opposite polarity after the driving operation is stopped under a state where the piezoelectric element A is charged, a sound is generated because of the following reason. Even if the driving operation is started while charging slow the piezoelectric element A in an initial state, the potential of the element is abruptly changed.
In order to reduce the level of such a sound, a method in which a piezoelectric element is gradually charged or discharged by using a drive pulse generating circuit has been proposed. As shown in FIG. 4, when application of a pulse voltage to the piezoelectric element A is stopped, for example, the bias voltage V.sub.HV may sometimes remain in the piezoelectric element A. As shown in FIG. 4, therefore, short pulses of the H signal are supplied to SLOW_F terminal after the driving operation is stopped, so as to intermittently operate the constant-current circuit I2. As a result, a small amount of charges corresponding to the on time of the constant-current circuit I2 are removed away from the piezoelectric element A. When this operation is repeated, the piezoelectric element A is totally discharged. According to this configuration, the generation of a sound can be prevented from occurring, without adding extra components.
In the method, the bias voltage V.sub.HV is discharged by using the constant-current circuit I2, and hence the pulse signal must remain to be supplied to the circuit even before the driving operation of the moving member M is started or after the driving operation is stopped, thereby producing a disadvantage that the burden of a microcomputer is large and the control is complicated.